It is known that the inability to correctly identify breast cancer in women with glandular and dense breasts is in part due to the overlap in tissues at different depths that conceal contained masses. Dense breasts are most common in younger women. In this group the sensitivity of mammography is less. We propose to adapt a newly invented C-scan ultrasound technology to develop a tomographic system that potentially can improve the visualization of cancerous masses and abnormalities in dense breast tissues. The system proposed is a real-time C-scan transmission ultrasound camera. Unlike conventional ultrasound, the proposed system produces speckle-free images striking for their radiographic appearance. Conventional ultrasound creates images based on the speed of sound transmission through tissues. The proposed system creates its images based on the tissue attenuation of sound, a separate physical parameter that can provide new information about the breast since sound transmission speed and attenuation of sound are not closely correlated. In addition, the proposed system combines the advantages of traditional mammogram imaging with the ability of ultrasound to resolve soft tissue layers. Evaluation of the proposed system will be performed on three levels: a technical system evaluation using breast phantoms, in-vitro study using breast specimens, and an in-vivo study involving a population of a minimum of 50 patients. This project aims at the development of a novel clinically viable system tailored to imaging human breasts for screening, diagnostic, and biopsy procedures. We believe that the high quality, high-resolution images coupled with tomographic capability will decrease the number of breast cancers that are currently missed. Specific tasks completed as a part of R21 are (1) evaluation of the existing laboratory system with recommendations for adaptation to breast imaging and (2) fabrication of a clinical prototype suitable for use by clinical personnel. Tasks completed as a part of R33 are (1) verification of clinical viability through breast phantom, in-vitro, and in-vivo studies, (2) definition of clinical protocols for breast cancer screening and diagnosis, and (3) verification of suitability in image-guided breast tissue biopsy procedures.